Superabsorbent polymers having delayed water absorption characteristics

ABSTRACT

A superabsorbent polymer comprising a delayed absorption superabsorbent polymer having a free water absorbency property of absorbing less than about 3 grams of aqueous saline per gram of superabsorbent polymer in about 6 seconds, for a full particle size distribution of superabsorbent polymer ranging from about 40 micrometers to about 890 micrometers.

[0001] This is a Divisional Application that claims priority to U.S.application Ser. No. 09/602,852 filed Jun. 26, 2000 and ProvisionalApplication Serial No. 60/141,412 filed Jun. 29, 1999.

TECHNICAL FIELD

[0002] The present invention relates, in general, to absorbent polymersthat absorb aqueous liquids (such as water, blood, and urine). Moreparticularly, the present invention relates to superabsorbent polymers,namely polymers that absorb over 100 times their weight in water, whichsuperabsorbent polymers have unique characteristics of delayed waterabsorption, and a novel method for making such superabsorbent polymers.As is well known, superabsorbent polymers have many uses, particularlyin absorbent sanitary articles, such as disposable diapers, disposableadult incontinence garments, disposable sanitary napkins, and disposablebandages. The superabsorbent polymers of the present invention, due totheir delayed water absorption characteristics, are particularly usefulin the manufacture of a web of superabsorbent polymer and cellulosicfiber for use as a core composite in such sanitary articles, when theweb is made by the wet-laid process.

Definitions of Abbreviations

[0003] The following abbreviations are employed throughout thisspecification. Abbreviation Definition AUL absorbency under loadAll-PEGMA allyloxy polyethylene glycol methacrylate, a X-linking agentcm centimeter CRC centrifuge retention capacity X-linking cross-linkingEO-TMPTA ethoxylated trimethylol-propane triacrylate, a X-linking agentFWA free water absorption mg milligram mm millimeter ppm parts permillion psi pounds per square inch SAP superabsorbent polymer, a polymerthat absorbs over 50 times, more preferably over 75 times, even morepreferably over 100 times, its weight in water ABAH2,2′-azobis(2-amidino-propane) dihydrochloride, a polymerizationinitiator

BACKGROUND OF THE INVENTION

[0004] When superabsorbent technology was first developed, only a highswelling capacity on contact of the superabsorbent polymer with liquids,referred to as the free swelling capacity in accordance with the freewater absorption test (FWA), was the primary consideration. However, itwas later realized that the water-absorbing polymers when present in asanitary article, such as a diaper or incontinence garment, aresubjected to mechanical load caused by movements of the person wearingthe article. Thus, a new consideration arose in that the superabsorbentpolymer, in addition to having a high swelling capacity, should alsohave a high capability for retaining liquid in accordance with thecentrifuge retention capacity test (CRC) and a high absorbency underpressure in accordance with the absorbency under load test (AUL). A gooddiscussion of the test for AUL can be seen in published European PatentApplication No. 0 339 461 A1 (published Nov. 2, 1989; priority to U.S.Ser. Nos. 184,302 (Parent) and 334,260 (Continuation-in-Part), whichContinuation-in-Part has issued as U.S. Pat. No. 5,147,343) toKellenberger, assignor to Kimberly-Clark Corporation.

[0005] Published European Patent Application No. 0 437 816 A1 (publishedJul. 24, 1991; priority to U.S. Ser. No. 464,798) to Kim and Nielsen,assignors to Hoechst Celanese Corporation, shows the wet-laid processfor the manufacture of webs of superabsorbent polymer and cellulosicfiber. These webs are employed as core composites in disposable sanitaryarticles, such as those mentioned above. More particularly, disclosed isa process that involves blending superabsorbent polymer particulateswith a liquid to form a slurry, followed by mixing cellulosic fiberswith the slurry and then filtering to remove part of the liquid, andfinally drying the resultant. The wet-laid process is also described inU.S. Pat. No. 4,605,401 (issued Aug. 12, 1986) to Chmelir and Künschner,assignors to Chemische Fabrik Stockhausen GmbH.

[0006] The journal article, “Keeping Dry with Superabsorbent Polymers”,Chemtech, (September, 1994) by Buchholz, contains an excellentdiscussion of the conventional methods for making superabsorbentpolymers, certain of which have sulfonate functional groups and certainof which have carboxylic acid functional groups. Also, Buchholzdiscussed various uses for superabsorbent polymers, such as in theabove-noted sanitary articles, as well as in a sealing composite betweenconcrete blocks that make up the wall of underwater tunnels and in tapesfor water blocking in fiber optic cables and power transmission cables.

[0007] A good discussion of the methods for making superabsorbentpolymers can also be seen in U.S. Pat. No. 5,409,771 (issued Apr. 25,1995) to Dahmen and Mertens, assignors to Chemische Fabrik StockhausenGmbH. More specifically, this patent mentions that commerciallyavailable superabsorbent polymers are generally cross-linked polyacrylicacids or cross-linked starch-acrylic-acid-graft-polymers, the carboxylgroups of which are partially neutralized with sodium hydroxide orcaustic potash. Also mentioned is that the superabsorbent polymers aremade by two methods, one being the solvent polymerization method and theother being the inverse suspension or emulsion polymerization method.

[0008] In the solvent polymerization method, an aqueous solution ofpartially neutralized acrylic acid for instance and a multi-functionalnetwork cross-linking agent is converted to a gel by radicalpolymerization. The resultant is dried, ground, and screened to thedesired particulate size.

[0009] On the other hand, in the inverse suspension or emulsionpolymerization method, an aqueous solution of partially neutralizedacrylic acid for instance is dispersed in a hydrophobic organic solventby employing colloids or emulsifiers. Then, the polymerization isstarted by radical initiators. Water is azeotropically removed from thereaction mixture after completion of the polymerization, followed byfiltering and drying the resultant product. Network cross-linkingtypically is accomplished by dissolving a polyfunctional cross-linkingagent in the monomer solution.

[0010] Furthermore, U.S. Pat. No. 5,154,713 (issued Oct. 13, 1992) toLind and U.S. Pat. No. 5,399,591 (issued Mar. 21, 1995) to Smith andLind, both of which patents are assigned to Nalco Chemical Company,describe new processes for making superabsorbent polymers, as a resultof which the superabsorbent polymers display an increased, faster waterabsorption. The superabsorbent polymers are depicted as useful asabsorbents for water and/or for aqueous body fluids when the polymersare incorporated into absorbent structures, such as disposable diapers,adult incontinence garments, and sanitary napkins.

[0011] General background with respect to various superabsorbentpolymers and methods of manufacturing them can be seen in U.S. Pat. No.5,229,466 (issued Jul. 20, 1993) to Brehm and Mertens; U.S. Pat. No.5,408,019 (issued Apr. 18, 1995) to Mertens, Dahmen, and Brehm; and U.S.Pat. No. 5,610,220 (issued Mar. 11, 1997) to Klimmek and Brehm, all ofwhich patents are assigned to Chemische Fabrik Stockhausen GmbH.

[0012] The disclosures of all above-mentioned patents and publishedpatent applications are incorporated herein by reference.

BRIEF SUMMARY AND OBJECTS OF THE INVENTION

[0013] Accordingly, the present invention provides a delayed absorption,particulate superabsorbent polymer comprising polymeric particles havinga free water absorption property of absorbing less than about 3 grams ofwater per gram of polymeric particle in about 6 seconds, for a fullparticle size distribution from about 40 to about 890 micrometers.

[0014] Also, the present invention provides a method for making suchsuperabsorbent polymers having the free water absorption propertydescribed in the paragraph above, wherein the method comprises a firststep of preparing a particulate superabsorbent polymer by conventionalmethods, followed by a second step of subjecting the resultantparticulate polymeric particles to a two-part thermal profile.Preferably, the two-part thermal profile comprises (a) heating thepolymeric particles for about 30 to about 90 minutes at a temperaturethat increases during the heating from a beginning temperature betweenabout 50 and about 80° C. to a final temperature between about 170 andabout 220° C., followed by (b) maintaining the resultant, heatedpolymeric particles from (a) for about 30 to about 90 minutes at aconstant temperature between about 5 and about 50° C. higher than thefinal temperature of (a).

[0015] Additionally, the present invention provides a wet-laid webcomprising a fibrous component and a component of the delayed absorptionsuperabsorbent polymers described in the two paragraphs above.Furthermore, the present invention provides a method for improving thesolids content of a wet-laid web by making the web with the delayedabsorption superabsorbent polymers described in the two paragraphsabove.

[0016] Therefore, it is an object of the present invention to provide asuperabsorbent polymer having a decreased, slower free water absorptionas compared to prior art superabsorbent polymers of similar particulatesize, which typically have a free water absorption of more than 5 gramsof water per gram of polymeric particles at 6 seconds, often more than 7grams of water per gram of polymeric particles at 6 seconds, and incertain instances, more than 20 grams of water per gram of polymericparticles at 6 seconds.

[0017] Furthermore, it is an advantage of the present delayedabsorption, superabsorbent polymers that they have not only anacceptable absorbency under load but also an improved solids content, asa result of which they are very useful in a wet-laid web ofsuperabsorbent polymer and cellulosic fiber for use as a core compositein sanitary articles.

[0018] Moreover, it is another advantage that due to the decreased freewater absorption property of the present superabsorbent polymers, theyare particularly useful in making a web by the wet-laid process sincethe decreased free water absorption should lead to less water uptakeduring the wet-laid process of blending an aqueous slurry ofsuperabsorbent polymer and cellulosic fiber, which in turn, should leadto less drying time of the resultant web prior to placing it as a corecomposite in the end product, such as a disposable diaper, a disposableadult incontinence garment, or a disposable sanitary napkin.

[0019] Additionally, one more advantage is that the presentsuperabsorbent polymers have an ultimate free water absorption property(i.e., the total amount of water absorbed when the superabsorbentpolymer is allowed to remain long enough, usually 3 to 5 minutes, inwater until no more water can be absorbed) that is essentially similarto that of prior art superabsorbent polymers, and consequently, thepresent superabsorbent polymers are just as absorbent as those of theprior art.

[0020] Some of the objects and advantages of the invention having beenstated, other objects and advantages will become evident as thedescription proceeds, when taken in connection with the LaboratoryExamples described below.

DETAILED DESCRIPTION OF THE INVENTION

[0021] As long as the above-mentioned two-part thermal profile isperformed on particulate superabsorbent polymer, the particulatesuperabsorbent polymer may be manufactured by any of the prior artprocesses for making superabsorbent polymers. For instance, thesuperabsorbent polymer may be made by the solvent polymerizationtechnique or may be made by the inverse suspension or emulsionpolymerization technique, which are well known techniques as discussedabove.

[0022] Thus, the superabsorbent polymer may be obtained by polymerizingat least about 10%, more preferably about 25%, and even more preferablyabout 55 to about 99.9%, by weight of monomers havingolefinically-unsaturated carboxylic and/or sulfonic acid groups. Suchacid groups include, but are not limited to, acrylic acids, methacrylicacids, 2-acrylamido-2-methylpropane sulfonic acid, and mixtures thereof.The acid groups are present as salts, such as sodium, potassium, orammonium salts.

[0023] The acid groups are typically neutralized to at least about 25mol %. Preferably, the extent of neutralization is to at least about 50mol %. More particularly, the preferred superabsorbent polymer has beenformed from cross-linked acrylic acid or methacrylic acid, which hasbeen neutralized to an extent of about 50 to about 80 mol %.

[0024] Additional useful monomers for making the superabsorbent polymersinclude from above 0 up to about 60% by weight of acrylamide,methacrylamide, maleic acid, maleic anhydride, esters (such ashydroxyethyl acrylate, hydroxyethylmethacrylate,hydroxypropylmethacrylate, glycidylmethacrylate, anddimethyl-aminoalkyl-methacrylate), dimethyl-aminopropyl acrylamide, andacrylamidopropyl trimethyl-ammonium chloride. Percentages below about60% of these monomers are desirable as percentages above about 60%typically will have a detrimental effect and deteriorate the swellcapacity of the resultant superabsorbent polymer.

[0025] Suitable network cross-linking agents useful in making thesuperabsorbent polymers are those which have at least two ethylenicallyunsaturated double bonds, those which have one ethylenically unsaturateddouble bond and one functional group reactive toward acid groups, andthose which have several functional groups reactive toward acid groups.Suitable kinds of network cross-linking agents include, but are notlimited to, acrylate and methacrylate of polyols (such as butanedioldiacrylate, hexanediol dimethacrylate, polyglycol diacrylate,trimethylolpropane triacrylate, tetrahydrofurfuryl-2-methacrylate,glycerol dimethacrylate, allyloxy polyethylene glycol methacrylate, andethoxylated trimethylolpropane triacrylate), allyl acrylate, dially)acrylamide, triallyl amine, diallyl ether, methylenebisacrylamide,N,N-dimethylaminoethylmethacrylate, N-dimethylaminopropylmethacrylamide, N-methylol methacrylamide, and N-methylolacrylamide.These network cross-linking agents are distinguished from and not to beconfused with the surface cross-linking agents discussed below.

[0026] Furthermore, depending on the desired end use, the superabsorbentpolymer may have a water-soluble polymeric component. The content mayrange from above 0 up to about 30% by weight of a component thatincludes, but is not limited to, partially or completely saponifiedpolyvinyl alcohol, polyvinyl pyrrolidone, starch, starch derivatives,polyglycols, polyacrylic acids, and combinations thereof. The molecularweight of the component is not critical, provided that it iswater-soluble. Preferred water-soluble polymeric components are starch,polyvinyl alcohol, and mixtures thereof. Preferably, the content of thewater-soluble polymeric component in the superabsorbent polymer rangesfrom about 1 to about 5% by weight, especially if starch and/orpolyvinyl alcohol are present as the water-soluble polymeric component.Also, the water-soluble polymeric component may be present as a graftpolymer having the acid-groups-containing polymer.

[0027] In connection with the particle shape of the superabsorbentpolymer, there are no specific limitations. The superabsorbent polymermay be in the form of small spheres obtained by inverse suspensionpolymerization, or in the form of irregularly shaped particles obtainedby drying and pulverizing the gel mass obtained by solventpolymerization. A typical particle size distribution ranges betweenabout 20 and about 2000 micrometers, preferably between about 40 andabout 890 micrometers, and more preferably between about 90 and about850 micrometers.

[0028] As is well known, the smaller the particle size, then the fastera superabsorbent polymer will absorb water, and likewise, the larger theparticle size, then the slower a superabsorbent polymer will absorbwater. Hence, for the present invention, the particulate superabsorbentpolymer desirably has the larger particle sizes, especially for use inmaking a core composite by the wet-laid process. Sizes under about 30micrometers are generally unsuitable for the wet-laid process.Nevertheless, for any given particle size, the superabsorbent polymer ofthe present invention should absorb less water in a selected amount ofseconds (i.e., exhibit a decreased, lower free water absorption) ascompared to a prior art superabsorbent polymer of essentially the sameparticle size.

[0029] In general, the prior art processing technique for themanufacture of superabsorbent polymers ends with a heat treatment. Thisis not to be confused with the special two-part thermal profile that iscritical in connection with manufacture of the superabsorbent polymersof the present invention so that they will have the desirably low freewater absorption characteristics.

[0030] More specifically, the following is noted with respect to thetwo-part thermal profile required for the present invention. The heatingof each of the two parts should be sufficient and the time of each ofthe two parts should be sufficient to achieve the inventivesuperabsorbent polymer with the desirable free water absorptionproperty, as described below.

[0031] In the first part, after the polymeric particles have been groundand then sieved to the appropriate, desirable size, they are heated bybeing subjected to an increasing temperature. Typically, this is atemperature starting at about 50° C., more preferably about 55° C., andeven more preferably about 60° C., and ending at about 170° C., morepreferably about 190° C., and even more preferably about 220° C. Then,for the second part, the temperature is quickly brought to at leastabout 5° C. higher than the ending temperature of the first part, andmaintained at that higher temperature. Preferably, the second partconstant temperature is no more than about 50° C. higher, morepreferably no more than about 30° C. higher, and even more preferably nomore than about 10° C. higher than the first part ending temperature.

[0032] The heating and the time for each of part one and part two of therequired two-part temperature profile should be sufficient so that theresultant superabsorbent polymeric particles exhibit a significantlyreduced free water absorption, as compared to prior art superabsorbentpolymeric particles of substantially the same particle size. Inparticular for the inventive particulate superabsorbent polymer, theslower free water absorption at about 6 seconds should be less thanabout 3 grams of water per gram of polymer, and in many instances, isless than about 2 grams of water per gram of superabsorbent polymer.

[0033] The free water absorption of the inventive superabsorbent polymeris referred to as delayed, reduced, or slower, as it is intended to meanthe free water absorption in a short amount of time, i.e., 6 seconds.This is distinguished from free water absorption where thesuperabsorbent polymer is allowed to absorb water until no more watercan be absorbed, which typically is 3 to 5 minutes, and is called theultimate free water absorption as a reference to the total amount ofwater absorbed regardless of how long that takes. The inventivesuperabsorbent polymers have an ultimate free water absorptionessentially the same as prior art superabsorbent polymers commerciallyused in sanitary articles.

[0034] A typical time for the first part of the temperature profileranges from about 30 minutes to about 90 minutes, more preferably fromabout 45 minutes to about 75 minutes, even more preferably from about 55minutes to about 65 minutes, and most preferably is about 60 minutes.Shorter times may be employed when higher temperatures are employed. Thetime for the second part of the required thermal profile is, in general,about the same as that for the first part, and likewise, shorter timesmay be employed with higher temperatures.

[0035] The superabsorbent polymers according to the present inventionmay be manufactured on a large scale by continuous or discontinuousprocesses.

[0036] Furthermore, the superabsorbent polymers according to the presentinvention may be used for a wide variety of applications, for instance,sanitary articles, water-blocking tapes and sheets for wherever leakingwater is a problem (i.e., inside of fiber-optic communication cables andpower transmission cables, between concrete blocks that make up thewalls of an underwater tunnel, such as the Channel Tunnel connectingEngland and France, as mentioned in the above-noted Buchholz journalarticle), and carriers for insecticides, pesticides and/or herbicides.

[0037] When the inventive superabsorbent polymers are used to make a webthat will be employed as a core composite in a sanitary article, theweight ratio of polymer component to fibrous component in the web shouldbe controlled to range from about 90:10 to about 5:95. A very suitableweb has a ratio from about 35:65 to about 45:55, and more preferably hasa ratio of about 40:60.

[0038] Although comminuted wood pulp (i.e., cellulosic fibers,colloquially referred to as fluff) is preferred to form the fibrouscomponent of the web for this invention, other wettable fibers such ascotton linters can be used. Additionally, the fibrous component may beformed from meltblown synthetic fibers such as polyethylene,polypropylene, polyesters, copolymers of polyesters and polyamides, andthe like. The fibrous component may also be formed from a mixture ofwood pulp fluff and one or more of the meltblown fibers. For example,the fibrous component may comprise at least about 5 weight % preferablyabout 10 weight % synthetic polymer fibers and the remainder maycomprise wood pulp fluff. The fibers of the web are generallyhydrophilic or rendered hydrophilic through a surface treatment.Cellulosic fiber is preferred, a preferred one being sold under thetrademark GOLDEN ISLES® by Georgia Pacific.

[0039] Especially, the inventive superabsorbent polymers, due to theirfree water absorption characteristics, are very useful in a wet-laidprocess for manufacturing a wet-laid web, having a superabsorbentpolymer component mixed with a fibrous component and useful as a corecomposite in a sanitary article. Examples of the wet-laid process aredescribed in the above-mentioned published European Patent ApplicationNo. 0 437 816 A1 and U.S. Pat. No. 4,605,401. As the wet-laid processinvolves mixing an aqueous slurry of superabsorbent polymer with fiber,water is absorbed during the wet-laid process. Consequently, at the endof the wet-laid process, the wet-laid web must be dried prior to placingit as a core composite in an end use article, such as a disposablediaper.

[0040] By employing the superabsorbent polymers of the presentinvention, less water should be absorbed during the wet-laid process ofmaking a web. Thus, there should be less water to remove during drying,resulting in a shorter drying time for the wetweb, which is veryadvantageous in a large scale factory production setting.

[0041] Moreover, after drying of the wet-laid web, due to the free waterabsorbency characteristics of the superabsorbent polymer, the web willhave an improved solids content, as compared to a wet-laid webcontaining prior art superabsorbent polymer. Typically, the inventivewet-laid web will have a solids content above about 18%.

[0042] Furthermore, the inventive superabsorbent polymers are wellsuited for use in a web, since they typically exhibit an acceptablecentrifuge retention capacity like that exhibited by prior artsuperabsorbent polymers. The inventive superabsorbent polymers usuallydisplay a centrifuge retention capacity of more than about 28, oftenmore than about 30, and even more than about 32 grams of aqueous salineper gram of superabsorbent polymer.

[0043] Additionally, the inventive superabsorbent polymers are wellsuited for use in a web, since they typically exhibit an acceptableabsorbency under load property, like that exhibited by prior artsuperabsorbent polymers. The inventive superabsorbent polymers usuallydisplay an absorbency under load property of more than about 13, oftenmore than about 15, and even more than about 18 grams of aqueous salineper gram of superabsorbent polymer.

[0044] As is known from the above-mentioned U.S. Pat. No. 5,409,771,coating a particulate superabsorbent polymer with an alkylene carbonatefollowed by heating to effect surface cross-linking improves theabsorbency under load characteristics. A desirable absorbency under loadproperty of at least about 13 grams of aqueous saline per gram ofsuperabsorbent polymer is especially desirable when the end use of thesuperabsorbent polymer is in a sanitary article, such as a disposablediaper, that is subjected to pressure from the person wearing thearticle.

[0045] Thus, the superabsorbent polymers of the present invention arepreferably coated with a surface X-linking agent prior to the inventivetwo-part thermal profile. The preferred alkylene carbonate for surfacecross-linking is ethylene carbonate.

[0046] More specifically, as described in U.S. Pat. No. 5,409,771, forcoating of particulate superabsorbent polymer with a surface X-linkingagent, the polymer may be mixed with an aqueous-alcoholic solution ofthe alkylene carbonate surface X-linking agent. The amount of alcohol isdetermined by the solubility of the alkylene carbonate and is kept aslow as possible for technical reasons, for instance, protection againstexplosions. Suitable alcohols are methanol, ethanol, butanol, or butylglycol, as well as mixtures of these alcohols. The preferred solvent iswater which typically is used in an amount of 0.3 to 5.0% by weight,relative to particulate superabsorbent polymer. In some instances, thealkylene carbonate surface X-linking agent is dissolved in water,without any alcohol. It is also possible to apply the alkylene carbonatesurface X-linking agent from a powder mixture, for example, with aninorganic carrier material, such as SiO₂.

[0047] To achieve the desired surface X-linking properties, the alkylenecarbonate has to be distributed evenly on the particulate superabsorbentpolymer. For this purpose, mixing is effected in suitable mixers, suchas fluidized bed mixers, paddle mixers, milling rolls, ortwin-worm-mixers. It is also possible to carry out the coating of theparticulate superabsorbent polymer during one of the process steps inthe production of the particulate superabsorbent polymer. A particularlysuitable process for this purpose is the inverse suspensionpolymerization process.

[0048] According to U.S. Pat. No. 5,409,771, the thermal treatment whichfollows the coating treatment is carried out as follows. In general, thethermal treatment is at a temperature between 150 and 300° C. However,if the preferred alkylene carbonates are used, then the thermaltreatment is at a temperature between 180 and 250° C. The treatmenttemperature depends on the dwell time and the kind of alkylenecarbonate. At a temperature of 150° C., the thermal treatment is carriedout for several hours. On the other hand, at a temperature of 250° C., afew minutes, e.g., 0.5 to 5 minutes, are sufficient to achieve thedesired surface X-linking properties. The thermal treatment may becarried out in conventional dryers or ovens. Examples of dryers andovens include rotary kilns, fluidized bed dryers, disk dryers, orinfrared dryers.

[0049] In contrast to the thermal treatment in U.S. Pat. No. 5,409,771,the present inventive thermal treatment (whether performed without orwith the presence of a surface X-linking agent) comprises theabove-described special two-part thermal profile. During the first part,the temperature is increased, and during the second part, thetemperature is maintained at a constant temperature at least about 5° C.higher, preferably no more than about 50° C. higher, than the endtemperature of the first part

[0050] To characterize the superabsorbent polymers as set out in theLaboratory Examples below (both those superabsorbent polymers of thepresent invention, as well as those comparison, superabsorbentpolymers), the centrifuge retention capacity (CRC), the absorbency underload (AUL), and the free water absorption (FWA) were measured in thefollowing manner.

[0051] CRC. The SAP's retention was determined according to the tea bagtest method and reported as an average value of two measurements.Approximately 200 mg of SAP, that have been sieved to a particle sizedistribution of 300 to 600 micrometers (not the indicated particle sizesin the Examples below), were enclosed in a tea bag and immersed in 0.9%by weight aqueous NaCl solution for 30 minutes. Then, the tea bag wascentrifuged at 1600 rpm for 3 minutes (centrifuge diameter was about 18cm) and weighed. Two tea bags without SAP were used as blanks.

[0052] Then, the CRC was calculated according to the following equation.${CRC} = \frac{W_{3} - W_{2} - W_{1}}{W_{1}}$

[0053] where:

[0054] CRC=Retention after an immersion time of 30 minutes (g of liquidabsorbed/g of SAP)

[0055] W₁=Initial Weight of SAP (g)

[0056] W₂=Weight of the average blank tea bags (without SAP) aftercentrifugation (g)

[0057] W₃=Weight of the tea bag with SAP after centrifugation (g)

[0058] AUL. The SAP's absorbency of a 0.9% by weight aqueous NaClsolution under load was determined according to the method described onpage 7 of the above-mentioned published European Patent Application No.0 339 461 A1. An initial weight of the SAP was placed in cylinder with asieve bottom. The SAP was loaded by a piston exerting a pressure load of60 g/cm². (It is noted 60 g/cm²≅0.9 psi.)

[0059] The cylinder was subsequently placed on aDemand-Absorbency-Tester (DAT) on a glass fritted disk of 125 mmdiameter, and covered by a Whatman filter paper #3. Then, the SAP wasallowed to absorb the 0.9% NaCl solution for 1 hour. The initial weightof the SAP was approximately 160 mg, which had been sieved to a particlesize distribution of 300 to 600 micrometers (not the indicated particlesizes in the Laboratory Examples below).

[0060] After the 1 hour, the swollen SAP was re-weighed, and grams ofthe 0.9% NaCl solution that had been retained was calculated. The AUL ofthe SAP was the grams retained.

[0061] FWA. To determine the SAP's free water absorption, a vacuumapparatus was assembled. More specifically, a vacuum pump was attached,by Tygon tubing, to a vacuum flask, atop which was positioned the bottomportion of a Buchner funnel, that was sealed properly to the flask usinga one-hole rubber stopper. A magnetic stirrer was placed beside theapparatus. After assemblage of the apparatus, the vacuum pump wasengaged and allowed to stay on throughout all FWA testing.

[0062] Using a 250 ml graduated cylinder, 150 ml±1 ml of 23.0° C.±0.5°tap H₂O was measured into a 250 ml beaker containing a 1 inch stir bar.The beaker of H₂O was placed on a stir plate and allowed to stir so thatthe created vortex ended approximately 2 to 3 cm from the surface of theliquid.

[0063] A dry, 80 mesh (180 micrometer) sieve was tared on a top loadingbalance, and then placed atop the Buchner funnel and tightly anchoredthrough suction. The SAP was then weighed on a separate balance in theamount needed for the particular test: the 30 second FWA determinationemployed 1 gram of SAP, while the 15 second and the 6 seconddeterminations each employed 3 grams of SAP. The SAP was poured into thebeaker of H₂O, while simultaneously a stopwatch was started to counttime from 0. When the SAP was poured into the tap H₂O, dispersion of thediscrete particles was immediate and complete in that no discreteparticles tended to clump or aggregate.

[0064] Upon reaching the number of seconds desired, the beaker contentswere poured into a sieve, with a transfer time of no greater than 3additional seconds. The sieve was left under the vacuum forapproximately 30 additional seconds. The sieve was then removed from theBuchner funnel, and wiped on its bottom surface of mesh to remove anyresidual H₂O. The dried sieve was then placed onto a previously taredbalance and the “Gel Weight” recorded.

[0065] Then, the FWA (g of liquid absorbed/g of SAP) was calculated fromthe gel weight according to the following equation.${{FWA}\left( {g\text{/}g} \right)} = \frac{{g\quad {Gel}\quad {Weight}} - {g\quad {Superabsorbent}}}{g\quad {Superabsorbent}}$

LABORATORY EXAMPLES I. Comparison Examples (of Commercially AvailableSAPs). Example A

[0066] Various commercially available, prior art superabsorbent polymerswere tested for FWA, CRC, and AUL. For the FWA test, each of the priorart superabsorbent polymers was tested at 27° C. at 750 rpm agitationspeed, and had a full particle size distribution of 44 to 841micrometers. For the CRC test and the AUL test, each of the prior artsuperabsorbent polymers was sieved so that tested was the above-notedparticle size distribution of 300 to 600 micrometers. The FWA test wasconducted with water, whereas each of the CRC test and the AUL test wasconducted with 0.9% by weight aqueous saline. The results are summarizedbelow in Table IA. TABLE IA Prior Art SAP and 6 15 30 Supplier secondsseconds seconds CRC AUL Company FWA (g/g) FWA (g/g) FWA (g/g) (g/g)(g/g) IM-4510 23.7 31.7 59.1 32.6 21.0 from Hoechst Celanese ASAP- 7.612.3 31.7 32.2 21.5 2300 from Chemdal Sumitomo- 10.8 20.8 50.9 36.9 9.660S from Sumitomo SalSorb- 8.9 14.8 23.1 36.5 11.9 CL20 from AlliedChemical FAVOR ® 5.6 11.5 18.7 36.5 21.0 SXM-77 from Stock- hausen

[0067] As can be seen, each prior art superabsorbent polymer exhibited aFWA at 6 seconds greater than 5 g/g.

Example B

[0068] Next, various selected particle size distributions ofStockhausen's FAVOR® SXM-77 were tested for FWA at 23° C. at 750 rpmagitation speed. The results are summarized below in Table IB. TABLE IBParticle Size (in Particle Size (in U.S. 15 seconds micrometers)standard meshes) FWA (g/g)  44 to 841  −20/+325 10.9 595 to 841 −20/+305.5 420 to 595 −30/+40 7.3 297 to 420 −40/+50 12.6 149 to 297  −50/+10026.1  88 to 149 −100/+170 53.6 44 to 88 −170/+325 73.3

[0069] As can be seen, only the largest particle size distribution (595to 841 micrometers) of superabsorbent SXM-77 exhibited a slow and lowFWA at 15 seconds of 5.5 g/g, which is in keeping with, as noted above,the inverse relationship that as the particle size increased, then theFWA decreased.

[0070] In contrast, as discussed in more detail below in LaboratoryExamples II A through H vis-a-vis superabsorbent polymers according tothe present invention, the full particle size distribution of 90 to 850micrometers for these superabsorbent polymers typically exhibited a FWAat 15 seconds of 4.0 g/g or less, and only one sample of this fullparticle size distribution exhibited a FWA at 15 seconds of 6.4 g/g.

II. Examples A through H (of SAPs of Present Invention) and ComparisonExamples A and B (of SAPs without Treatment of Two-Part ThermalProfile).

[0071] In the following examples, each superabsorbent polymer was across-linked sodium polyacrylate made by solvent polymerization. Also,each percentage recited was a weight %, unless specifically indicatedotherwise as a mol %, and the aqueous ethylene carbonate was a solutionof 50 parts by weight of ethylene carbonate and 50 parts by weight ofdeionized water.

Example A

[0072] An aqueous acrylic acid solution comprising 0.1% EO-TMPTA as across-linking agent, 0.25% AII-PEGMA as a co-cross-linking agent, and2.5% methoxy polyethylene glycol methacrylate, all relative to acrylicacid, was neutralized with sodium hydroxide solution under cooling. Theacrylic acid concentration of the monomer solution amounted to 29%, witha neutralization degree of 70 mol %.

[0073] The monomer solution was cooled to about 5° C., purged withnitrogen, and then mixed with sodium erythobate solution as a reducingagent, hydrogen peroxide solution as an oxidant, (the sodium erythobateforming a redox initiator couple with the hydrogen peroxide), sodiumcarbonate solution as a foaming agent to generate a porous polymer gel,and a fourth solution containing both ABAH and sodium persulfate asthermal initiators which generate free radicals throughout the course ofthe reaction to complete the polymerization. The final concentration ofeach of sodium erythobate, hydrogen peroxide, sodium carbonate, ABAH,and sodium persulfate was respectively at 57, 125, 600, 125, and 100ppm, all relative to total monomer solution.

[0074] Polymerization started immediately after the monomer solution wasmixed with all other solutions. After 20 minutes of polymerization, theformed polymer gel was crumbled and dried in hot air at 150° C. for 20minutes.

[0075] The dried polymer was subsequently ground, screened to 90 to 850micrometers and continuously fed into a paddle mixer (380 rpm) at 4000kg/hour while mixing with aqueous ethylene carbonate at a 1:167 ratio byweight of ethylene carbonate to polymer in order to coat this surfacecross-linking agent onto the polymer.

[0076] The mixture was then transferred to a conveyor where it washeated from a beginning temperature of 65° C. to a final temperature of185° C. within 1 hour for the first part of the thermal profile.Subsequently, the mixture was rapidly brought to 200° C. and maintainedat that constant temperature of 200° C. for an additional 45 minutes forthe second part of the thermal profile. After cooling, the resultantproduct was transported to a storage vessel.

Example B

[0077] The same procedure as described in Example A was used except thatfor the second part of the thermal profile, the mixture was maintainedfor 35 minutes at a constant temperature of 210° C. after thepolymer/ethylene carbonate mixture had been heated for the first part ofthe thermal profile to a final temperature of 185° C. The resultantproduct was transported to a storage vessel after cooling.

Example C

[0078] The same procedure as described in Example A was used except thatfor the second part of the thermal profile, the mixture was maintainedfor 50 minutes at a constant temperature of 205° C. after thepolymer/ethylene carbonate mixture had been heated for the first part ofthe thermal profile to a final temperature of 185° C. The resultantproduct was transported to a storage vessel after cooling.

Example D

[0079] An aqueous acrylic acid solution comprising 0.19% triallyl amineas a cross-linking agent, relative to acrylic acid, was neutralized withsodium hydroxide solution under cooling. The acrylic acid concentrationof the monomer solution amounted to 31%, with a neutralization degree of70 mol %.

[0080] The monomer solution was cooled to about 5° C., purged withnitrogen, and then mixed with sodium erythobate solution as a reducingagent, t-butyl hydrogen peroxide solution as an oxidant, (the sodiumerythobate forming a redox initiator couple with the t-butyl hydrogenperoxide), and a third solution containing both ABAH and sodiumpersulfate as thermal initiators which generate free radicals throughoutthe course of the reaction to complete the polymerization. The finalconcentration of each of sodium erythobate, t-butyl hydrogen peroxide,ABAH, and sodium persulfate was respectively at 26, 182, 195, and 100ppm, all relative to total monomer solution.

[0081] Polymerization started immediately after the monomer solution wasmixed with all other solutions. After 20 minutes of polymerization, theformed polymer gel was crumbled and dried in hot air at 150° C. for 20minutes.

[0082] The dried polymer was subsequently ground, screened to 90 to 850micrometers and continuously fed into a paddle mixer (380 rpm) at 4000kg/hour while mixing with aqueous ethylene carbonate as a surfacecross-linking agent at a 1:167 ratio by weight of ethylene carbonate topolymer in order to coat this surface cross-linking agent onto thepolymer.

[0083] The mixture was then transferred to a conveyor where it washeated from a beginning temperature of 80° C. to a final temperature of170° C. within 1 hour for the first part of the thermal profile.Subsequently, the mixture was maintained at a constant temperature of200° C. for an additional 60 minutes for the second part of the thermalprofile. After cooling, the resultant product was transported to astorage vessel.

Example E

[0084] The same procedure as described in Example D was used except thatfor the second part of the thermal profile, the mixture was maintainedfor an additional 60 minutes at a constant temperature of 205° C., afterthe polymer/ethylene carbonate mixture had been heated to the finaltemperature of 170° C. for the first part of the thermal profile. Theresultant product was transported to a storage vessel.

Example F

[0085] The same procedure as described in Example D was used except thatfor the second part of the thermal profile, the mixture was maintainedfor an additional 45 minutes at a constant temperature of 210° C. afterthe polymer/ethylene carbonate mixture had been heated to the finaltemperature of 170° C. for the first part of the thermal profile. Theresultant product was transported to a storage vessel after cooling.

Example G

[0086] An aqueous acrylic acid solution comprising 0.19% triallyl amineas a cross-linking agent, relative to acrylic acid, was neutralized withsodium hydroxide solution under cooling. The acrylic acid concentrationof the monomer solution amounted to 31%, with a neutralization degree of60 mol %.

[0087] The monomer solution was cooled to about 5° C., purged withnitrogen, and then mixed with ascorbic acid solution as a reducingagent, t-butyl hydrogen peroxide solution as an oxidant, (the ascorbicacid forming a redox initiator couple with the t-butyl hydrogenperoxide), and a third solution containing both ABAH and sodiumpersulfate. The final concentration of each of ascorbic acid, t-butylhydrogen peroxide, ABAH, and sodium persulfate was respectively at 22,178, 200, and 100 ppm, all relative to total monomer solution.

[0088] Polymerization started immediately after the monomer solution wasmixed with all other solutions. After 20 minutes of polymerization, theformed polymer gel was crumbled and dried in hot air at 150° C. for 20minutes.

[0089] The dried polymer was subsequently ground, screened to 90 to 850micrometers and continuously fed into a paddle mixer (380 rpm) at 4000kg/hour while mixing with aqueous ethylene carbonate as a surfacecross-linking agent, at a 1:206 ratio by weight of ethylene carbonate topolymer in order to coat this surface cross-linking agent onto thepolymer.

[0090] The mixture was then transferred to a conveyor where it washeated from a beginning temperature of 80° C. to a final temperature of175° C. within 1 hour for the first part of the thermal profile.Subsequently, the mixture was maintained at a constant temperature of180° C. for an additional 45 minutes for the second part of the thermalprofile. After cooling, the resultant product was transported to astorage vessel.

Example H

[0091] The same procedure as describe in Example G was used except thatfor the second part of the thermal profile, the mixture was maintainedfor an additional 35 minutes at a constant temperature of 190° C. afterthe polymer/ethylene carbonate mixture had been heated to the finaltemperature of 175° C. The resultant product was transported to astorage vessel after cooling.

Comparison Example A (with only First Part of Two-part Thermal Profile)

[0092] The polymer prepared as described in Example E was dried, ground,screened to 90 to 850 micrometers, and continuously fed into a paddlemixer (380 rpm) at 4000 kg/hour while mixing with ethylene carbonate asa surface cross-linking agent at a 1:167 ratio by weight of ethylenecarbonate to polymer in order to coat this surface cross-linking agentonto the polymer.

[0093] The mixture was then transferred to a conveyor where it washeated from a beginning temperature of 80° C. to a final temperature of175° C. within 1 hour as the first part of the thermal profile, and thenafter cooling, the resultant product was transferred to a storagevessel. The second part of the thermal profile was not performed.

Comparison Example B (with only Second Part of Two-part Thermal Profile)

[0094] The polymer prepared as described in Example E was dried, ground,screened to 90 to 850 micrometers, and continuously fed into a paddlemixer (380 rpm) at 4000 kg/hour while mixing with ethylene carbonate ata 1:167 ratio by weight of ethylene carbonate to polymer in order tocoat this surface cross-linking agent onto the polymer.

[0095] The mixture was then transferred to a conveyor where it washeated at a constant temperature of 205° C. for 2 hours for the secondpart of the thermal profile. The resultant product was cooled andtransported to a storage vessel. The first part of the thermal profilewas not performed.

[0096] The resultant superabsorbent polymers of Examples A through H andComparison Examples A and B were tested for FWA, CRC, and AUL. For theFWA test, the particle size distribution was the full 90 to 850micrometers. However, for the CRC test and the AUL test, the polymerswere sieved, and hence, the particle size distribution was theabove-noted 300 to 600 micrometers. The FWA test was conducted withwater, whereas each of the CRC test and the AUL test was conducted with9% by weight aqueous saline. The results are summarized below in TableII. TABLE II Example 6 15 30 of seconds seconds seconds CRC AUL SAP FWA(g/g) FWA (g/g) FWA (g/g) (g/g) (g/g) Ex. A 2.6 6.4 18.8 33.5 19.2 Ex. B1.9 4.0 14.0 30.3 21.6 Ex. C 1.3 3.0 11.3 28.4 19.7 Ex. D 0.9 2.8 10.033.0 15.6 Ex. E 1.1 2.4 9.3 35.3 13.7 Ex. F 1.2 2.7 8.3 31.9 14.7 Ex. G1.5 3.3 9.7 30.0 18.2 Ex. H 1.6 3.6 10.2 29.4 20.3 Compari- 5.7 13.522.4 38.3 11.5 son A Compari- 3.8 11.4 35.2 37.9 10.0 son B

[0097] As can be seen, for the inventive superabsorbent polymers thathad been subjected to the two-part thermal profile, each exhibited a FWAat 6 seconds less than 3 g/g, and most exhibited a FWA at 6 seconds lessthan 2 g/g. On the other hand, for the two comparisons that had beensubjected to only one of the two parts of the thermal profile, eachexhibited a FWA at 6 seconds greater than 3.5 g/g. Moreover, each of thesuperabsorbent polymers that had been subjected to the two-part thermalprofile exhibited a far superior AUL, as compared to the AUL of each ofthe two comparisons.

III. Examples of Web of SAP and Cellulosic Fluff made by Wet-LaidProcess

[0098] In the following examples, selected inventive SAPs and also thetwo comparison SAPs, made as described above in Example II, were eachrespectively employed in a wet-laid process to make a wet-laid web ofSAP and cellulosic fiber.

[0099] More specifically, 1.36 grams of cellulosic fiber (GOLDENISLES®4800 sold by Georgia Pacific) was added to 200 grams of tap water,and then, 0.9 gram of the selected SAP was added. The resultant slurrywas then poured into a laboratory web molder having a 150 micrometerpolyester screen at the bottom.

[0100] The web molder was made with a stainless steel, sampling chamberon the top for retaining the slurry. The chamber measured 8.5 cm indiameter and 10 cm in height. Also, the web molder had a bottom sectionthat was connected through a ball valve to a vacuum system.

[0101] The slurry was agitated with a 3-blade fan-shaped turbineagitator moving in an up-and-down fashion for 5 times. The watertemperature was controlled at 23° C.±1° C., and the total water contacttime of the SAP and cellulosic fiber mixture was controlled to be 10seconds. Next, the water was drained under vacuum (60 mm Hg) from theslurry, with a draining time of 60 seconds.

[0102] The solids content of each respective web was determinedaccording to the following equation:

[0103] Solids wt %=[(fiber wt+SAP wt)/web wt]×100% where each wt (i.e.,the weight of fiber, the weight of SAP, and the weight of web) was ingrams. The results are summarized below in Table III. TABLE IIISAP/Cellulosic Fiber Web Solids Content Example of SAP Ratio(weight/weight) (weight %) none 0/100 23.9 Example A 40/60 18.1 ExampleC 40/60 21.6 Example D 40/60 22.1 Example E 40/60 23.9 Example F 40/6023.0 Example H 40/60 22.1 Comparison A 40/60 16.3 Comparison B 40/6017.7

[0104] As can be seen from the above Table III, wet-laid webs made withthe inventive SAPs (Examples A, C, D, E, F, and H) exhibited an improvedsolids content versus wet-laid webs made with the comparison SAPs(Comparison Examples A and B). More specifically, the solids content foreach of the wet-laid webs made with the inventive SAPs was always above18%, whereas the solids content for each of the wet-laid webs made withthe comparison SAPs was always below 18%.

[0105] It will be understood that various details of the invention maybe changed without departing from the scope of the invention.Furthermore, the foregoing description is for the purpose ofillustration only, and not for the purpose of limitation—the inventionbeing defined by the claims.

What is claimed is:
 1. A superabsorbent polymer comprising a delayedabsorption, particulate superabsorbent polymer having a free waterabsorption property of absorbing less than about 3 grams of water pergram of superabsorbent polymer in about 6 seconds, for a full particlesize distribution ranging from about 40 micrometers to about 890micrometers.
 2. The delayed absorption, particulate superabsorbentpolymer of claim 1, wherein the superabsorbent polymer has a free waterabsorption property of absorbing less than about 7 grams of water pergram of superabsorbent polymer in about 15 seconds, for a full particlesize distribution ranging from about 40 micrometers to about 890micrometers.
 3. The delayed absorption, particulate superabsorbentpolymer of claim 1, wherein the superabsorbent polymer has a centrifugeretaining capacity property of retaining more than 28 grams of aqueoussaline per gram of superabsorbent polymer.
 4. The delayed absorption,particulate superabsorbent polymer of claim 1, wherein thesuperabsorbent polymer has an absorbency under load property at 0.9 psi(60 g/cm²) of retaining more than 13 grams of aqueous saline per gram ofsuperabsorbent polymer.
 5. The delayed absorption, particulatesuperabsorbent polymer of claim 1, wherein the superabsorbent polymer issurface cross-linked.
 6. A method for making a delayed absorption,particulate superabsorbent polymer, said method comprising: (a)preparing a particulate superabsorbent polymer, and (b) subjecting theparticulate superabsorbent of step (a) to a thermal profile having afirst part and a second part, wherein the first part comprises heatingthe superabsorbent polymer with an increasing temperature from abeginning temperature to a final increased temperature and the secondpart comprises maintaining heating of the superabsorbent polymer at aconstant temperature that is at least about 5° C. higher than the finalincreased temperature of the first part, and wherein the temperaturesand the times for heating of each of the first part and the second partare sufficient to achieve a delayed absorption, particulatesuperabsorbent polymer having a free water absorption property ofabsorbing less than about 3 grams of aqueous saline per gram ofsuperabsorbent polymer in about 6 seconds, for a full particle sizedistribution ranging from about 40 micrometers to about 890 micrometers.7. The method of claim 6, wherein the constant temperature formaintaining the heating in the second part is a constant temperaturefrom about 5° C. to about 50° C. higher than the final increasedtemperature of the first part.
 8. The method of claim 6, wherein thetemperature of the first part increases from a beginning temperaturebetween about 50° C. and about 80° C. to a final increased temperaturebetween about 170° C. and about 220° C., and the constant temperature inthe second part is between about 175° C. and about 270°.
 9. The methodof claim 6, wherein the time for the heating of the first part rangesfrom about 30 minutes to about 90 minutes.
 10. The method of claim 6,wherein the time for the heating of the second part ranges from about 30minutes to about 90 minutes.
 11. The method of claim 6, wherein thesuperabsorbent polymer has a free water absorption property of absorbingless than about 6 grams of water per gram of superabsorbent polymer inabout 15 seconds, for a full particle size distribution ranging fromabout 40 micrometers to about 890 micrometers.
 12. The method of claim6, wherein the superabsorbent polymer has a centrifuge retentioncapacity property of retaining more than 28 grams of aqueous saline pergram of superabsorbent polymer.
 13. The method of claim 6, wherein thesuperabsorbent polymer has an absorbency under load property ofretaining more than 13 grams of aqueous saline per gram ofsuperabsorbent polymer.
 14. The method of claim 6, wherein step (a)includes a coating treatment with a surface cross-linking agent and step(b) is performed after the coating treatment.
 15. A wet-laid webcomprising a fibrous component and a superabsorbent polymer component,wherein: (a) the superabsorbent polymer comprises a delayed absorption,particulate superabsorbent polymer having a free water absorptionproperty of absorbing less than about 3 grams of water per gram ofsuperabsorbent polymer in about 6 seconds, for a full particle sizedistribution ranging from about 40 micrometers to about 890 micrometers;and (b) the weight ratio of the superabsorbent polymer component to thefibrous component is controlled to be in a range from about 90:10 toabout 5:95.
 16. The wet-laid web of claim 15, wherein the superabsorbentpolymer has a free water absorption property of absorbing less thanabout 7 grams of water per gram of superabsorbent polymer in about 15seconds, for a full particle size distribution ranging from about 40micrometers to about 890 micrometers.
 17. The wet-laid web of claim 15,wherein the superabsorbent polymer has a centrifuge retention capacityproperty of retaining more than 28 grams of aqueous saline per gram ofsuperabsorbent polymer.
 18. The wet-laid web of claim 15, wherein thesuperabsorbent polymer has an absorbency under load property at 0.9 psi(60 g/cm²) of retaining more than 13 grams of aqueous saline per gram ofsuperabsorbent polymer.
 19. The wet-laid web of claim 15, wherein thesuperabsorbent polymer is surface cross-linked.
 20. A method forimproving the solids content of a wet-laid web having a fibrouscomponent and a superabsorbent polymer component, said methodcomprising: (a) preparing a particulate superabsorbent polymer; (b)forming the superabsorbent polymer component by subjecting theparticulate superabsorbent of step (a) to a thermal profile having afirst part and a second part, wherein the first part comprises heatingthe superabsorbent polymer with an increasing temperature from abeginning temperature to a final increased temperature and the secondpart comprises maintaining heating of the superabsorbent polymer at aconstant temperature that is at least about 5° C. higher than the finalincreased temperature of the first part, and wherein the temperaturesand the times for heating of each of the first part and the second partare sufficient to achieve a delayed absorption, particulatesuperabsorbent polymer having a free water absorption property ofabsorbing less than about 3 grams of aqueous saline per gram ofsuperabsorbent polymer in about 6 seconds, for a full particle sizedistribution ranging from about 40 micrometers to about 890 micrometers;(c) forming an aqueous suspension of the fibrous component together withthe superabsorbent polymer component from step (b); and (d) drying thesuspension from step (c) to achieve a wet-laid web having an improvedsolids content.
 21. The method of claim 20, wherein the constanttemperature for maintaining the heating in the second part is a constanttemperature from about 5° C. to about 50° C. higher than the finalincreased temperature of the first part.
 22. The method of claim 20,wherein the temperature of the first part increases from a beginningtemperature between about 50° C. and about 80° C. to a final increasedtemperature between about 170° C. and about 220° C., and the constanttemperature in the second part is between about 175° C. and about 270°C.
 23. The method of claim 20, wherein the time for the heating of thefirst part ranges from about 30 minutes to about 90 minutes.
 24. Themethod of claim 20, wherein the time for the heating of the second partranges from about 30 minutes to about 90 minutes.
 25. The method ofclaim 20, wherein the superabsorbent polymer has a free water absorptionproperty of absorbing less than about 6 grams of water per gram ofsuperabsorbent polymer in about 15 seconds, for a full particle sizedistribution ranging from about 40 micrometers to about 890 micrometers.26. The method of claim 20, wherein the superabsorbent polymer has acentrifuge retention capacity property of retaining more than 28 gramsof aqueous saline per gram of superabsorbent polymer.
 27. The method ofclaim 20, wherein the superabsorbent polymer has an absorbency underload property of retaining more than 13 grams of aqueous saline per gramof superabsorbent polymer.
 28. The method of claim 20, wherein step (a)includes a coating treatment with a surface cross-linking agent and step(b) is performed after the coating treatment.
 29. The method of claim20, wherein the resultant wet-laid web has an improved solids contentabove about 18 weight %.